1. Field of the Invention
The present invention relates to an electrophotographic developing device for developing a latent image formed on a photosensitive drum with use of a toner as a developer, and more particularly to an electrophotographic single-component developing device including a toner storing portion for storing a single-component toner supplied from a toner container, and an agitator for feeding the toner from the toner container to the toner storing portion and supplying the toner from the toner storing portion to a development roller.
2. Description of the Related Art
FIGS. 9 and 10 show a prior art electrophotographic single-component developing device using a single-component toner as a developer. Referring to FIGS. 9 and 10, reference numeral 1 designates a housing of the developing device. There is defined in the housing 1 a toner storing portion 2 for storing a magnetic single-component toner TN. The toner storing portion 2 is connected to a toner supply passage 3 into which the toner TN is supplied from a toner hopper (not shown). Toner feeding means 4 for feeding the toner TN is provided in the toner storing portion 2 and the toner supply passage 3.
As shown in FIG. 11, the toner feeding means 4 comprises a rotating shaft 4a, a spiral blade 4b formed on the rotating shaft 4a, and a U-shaped agitator rod 4c consisting of a pair of vertical portions 4c1 connected to the rotating shaft 4a and a horizontal portion 4c2 extending between the vertical portions 4c1 in parallel relationship to the rotating shaft 4a. The spiral blade 4b is located in the toner supply passage 3, and the agitator rod 4c is located in the toner storing portion 2. The rotating shaft 4a of the toner feeding means 4 is rotated in a direction depicted by an arrow P in FIG. 9, and the toner TN supplied from the toner hopper to the toner supply passage 3 is accordingly fed into the toner storing portion 2 by the rotation of the spiral blade 4b. The agitator rod 4c has no ability to feed the toner TN in the toner storing portion 2 in the axial direction of the rotating shaft 4a. Therefore, the toner TN is stored into the whole of the toner storing portion 2 only by the ability of toner feed of the spiral blade 4b.
The toner TN stored in the toner storing portion 2 is supplied to a development roller 5 by the rotation of the agitator rod 4c. The toner TN supplied to the development roller 5 is magnetically attached to a development sleeve 6 rotating in a direction depicted by an arrow Q in FIG. 9. At this time, the rising amount (height) of a magnetic brush of the toner TN on the development sleeve 6 is restricted by a blade 7 to form a uniform thin layer on the development sleeve 6. Then, the toner TN in the form of the uniform thin layer on the development sleeve 6 is transferred to a photosensitive drum 8 rotating in a direction depicted by an arrow R. Thus, an electrostatic latent image formed on the photosensitive drum 8 is developed by the toner TN.
In the above developing device, the amount of the toner TN to be used for the development of an image having a normal image density is not so large. Accordingly, the toner TN is supplied into the whole of the toner storing portion 2, that is, over the development roller 5 only by the ability of toner feed of the spiral blade 4b.
However, in continuously developing an image having a high image density (e.g., 20% or more) such as a photograph image, the amount of the toner TN to be used becomes very large. Accordingly, the amount of the toner TN is supplied only by the ability of toner feed of the spiral blade 4b and becomes insufficient. That is, in continuously using a large amount of the toner TN, the conventional developing device is inferior in the ability of toner feed in a direction parallel to the axis of the development roller 5, thus causing a problem such that the toner TN cannot be sufficiently and uniformly supplied over the toner storing portion 2, i.e., over the development roller 5 to and thus outputs a partially lacking developed image.
Further, the conventional developing device has a structure such that the toner TN is not circulated in the toner storing portion 2. Accordingly, a part of the toner TN not consumed in developing an image having a normal image density is deteriorated in the toner storing portion 2. The deteriorated toner TN is gradually gathered at an end portion of the toner storing portion 2 in the feeding direction as time proceeds. The gathering of the deteriorated toner TN hinders uniform supply of the toner TN in the axial direction of the development roller 5, causing a reduction in quality of the developed image such that a uniform density of the toner image cannot be obtained even in developing an image having a normal image density.
As another conventional developing device using a two-component developer, there is disclosed in U.S. Pat. No. 4,980,724 a two-component developing device including a spiral blade with a paddle extending in an axial direction of an agitator, and there is also disclosed in Japanese Patent Laid-open Publication No. 3-168781 a two-component developing device including a spiral blade with an agitating blade extending in parallel to a rotating shaft and further including a spiral powder feeding groove. Each two-component developing device has realized a relatively high ability of feed of a toner or a two-component developer owing to the spiral blade or the combination of the spiral blade and the spiral powder feeding device as developer feeding means.
It is accordingly considered to apply the developer feeding means of the two-component developing device having the relatively high feed ability mentioned above to the toner feeding means of the single-component developing device mentioned above, so as to enhance the toner feed ability of the single-component developing device.
However, since each two-component developing device mentioned above uses a two-component developer, it is necessary to agitate a toner and a carrier. Therefore, the developer feeding means of each two-component developing device is set to have the ability of very strong agitation of the developer. For this reason, the application of the developer feeding means of each two-component developing device to the toner feeding means of the single-component developing device will cause vigorous agitation of the toner, so that the toner may be greatly deteriorated to reduce the image quality. Consequently, the developer feeding means of each two-component developing device cannot be applied to the toner feeding means of the single-component developing device.